1. Field of the Invention
This invention relates in general to processing semiconductor wafers, and, in particular, to a method and apparatus for rapid thermal processing of a plurality of semiconductor wafers simultaneously and of a single large semiconductor wafer.
2. Related Art
Deposition of a film on the surface of a semiconductor wafer is a common step in semiconductor processing. Typically, selected chemical gases are mixed in a deposition chamber containing a semiconductor wafer. Usually, heat is applied to drive the chemical reaction of the gases in the chamber and to heat the surface of the wafer on which the film is deposited.
In deposition processes, it is desirable to maximize wafer throughput (i.e., the number of wafers processed per unit time), while depositing film layers that have uniform thickness and resistivity. To obtain uniform thickness and resistivity, it is important to maintain the wafer at a uniform temperature.
A number of different deposition reactors have been developed. Generally, each deposition reactor has a reaction chamber, a wafer handling system, a heat source and temperature control, and a gas delivery system (inlet, exhaust, flow control).
FIG. 1A is a simplified cross-sectional view of one type of prior art deposition reactor 100 known as a horizontal furnace, in which susceptor 101 is positioned in horizontal tube 102 (usually of rectangular cross-section), the interior of which is the reaction chamber. Semiconductor wafers 103a, 103b, and 103c are mounted on surface 101a of susceptor 101. Heat source 104 heats the wafers, and reactant gases 105 are flowed through tube 102 past the wafers. Susceptor 101 is often tilted, as shown in FIG. 1A, so that surface 101a faces into the flow of reactant gases 105 to minimize the problem of reactant depletion in the vicinity of the wafers near the end of the flow of reactant gases 105.
FIG. 1B is a simplified orthogonal view of another type of prior art reactor 110, known as a barrel reactor, in which susceptor 111 is suspended in the interior of bell jar 112 which defines the reaction chamber. Semiconductor wafers, e.g., wafer 113, are mounted substantially vertically on the sides, e.g., side 111a, of susceptor 111. Heat source 114 heats the wafers, and reactant gases are introduced through gas inlet 115 into the top of bell jar 112. The gases pass down the length of susceptor 111 , over the surfaces of the wafers, and are exhausted from the reaction chamber through a gas outlet (not shown) at the bottom of bell jar 112.
FIG. 1C is a simplified cross-sectional view of yet another type of prior art conventional chemical vapor deposition reactor 120, known as a pancake reactor, in which vertically fixed susceptor 121 is supported from the bottom of bell jar 122 which defines the reaction chamber. Semiconductor wafers, e.g., wafer 123, are mounted horizontally on surface 121a of susceptor 121. The wafers are heated by a RF heat source (not shown), and reactant gases are introduced into the reaction chamber above the wafers through susceptor support 125. The gases flow down over the wafers and are exhausted through a gas outlet (not shown) at the bottom of bell jar 122.
Deposition reactors may be classified according to characteristics of their operation. For instance, a reactor may be either cold wall or hot wall. Cold wall reactors are usually preferred because undesirable deposits do not build up on the chamber walls.
A reactor may also be characterized by the amount of time that is required to heat up and cool down the wafer. Conventional reactors take on the order of 40-90 minutes for a complete process cycle of a batch of wafers. Rapid thermal process (RTP) reactors, on the other hand, require only 2-15 minutes to process a wafer. Thus, rapid thermal reactors are characterized by the fact that the process cycle time is significantly less than the process cycle time for a conventional reactor.
Conventional reactors have been used to process a plurality of wafers or a single wafer in one batch, while RTP reactors have been used to process single wafer batches. RTP reactors have not been used for processing multiple wafer batches because the rapid temperature changes in RTP reactors make it difficult to achieve a uniform temperature area in the reaction chamber. The area of the reaction chamber with a uniform temperature limits the operation to a single wafer, typically with a diameter of 200 mm (8 inches) or less.
While RTP reactors have been used to process one wafer at a time, as opposed to the multiple wafer processing of conventional reactors, the one wafer batch capacity of the RTP reactor has been acceptable only because these reactors achieve more uniform resistivities and thicknesses than possible with conventional reactors. In conventional reactors, thickness and resistivity variations of 3-10% are achievable. In RTP reactors, thickness variations of 1-2% and resistivity variations of 1-5% are achievable.
A reactor may also be characterized according to the orientation of the wafer in the reaction chamber. A vertical reactor is one in which the surface on which gases are deposited is substantially vertical. A horizontal reactor is one in which the surface on which gases are deposited is substantially horizontal.
A reactor may also be characterized according to the type of heat source used to heat the wafers. Use of radiant heating for semiconductor processing is known in the prior art and relates back to the late sixties. A variety of systems have been developed for semiconductor processing which include either a radiant energy heat source or a RF energy heat source and a susceptor. However, each of these apparatus' suffer from one or more problems.
Sheets, U.S. Pat. No. 4,649,261 entitled "Apparatus for Heating Semiconductor Wafers in Order To Achieve Annealing, Silicide Formation, Reflow of Glass, Passivation Layers, etc.", used two radiant heat sources--a continuous wave and a pulsed heat source--to heat a stationary wafer at 200.degree. C. to 500.degree. C. per second. Shimizu, U.S. Pat. No. 4,533,820 entitled "Radiant Heating Apparatus", shows a reaction chamber surrounded by a plurality of planar light sources which heat a semiconductor wafer supported by a pedestal. Shimizu reported that a uniform oxide film was formed on the semiconductor wafer within three minutes after the lights were turned-on.
Other configurations using dual radiant heat sources to heat a semiconductor wafer are shown, for example, in U.S. Pat. No. 4,680,451, entitled "Apparatus Using High Intensity CW Lamps for Improved Heat Treating of Semiconductor Wafer," issued to Gat et al. on Jul. 14, 1987 and U.S. Pat. No. 4,550,245, entitled "Light-Radiant Furnace for Heating Semiconductor Wafers," issued to Arai et al., on Oct. 29, 1985. Gat et al. reported heating a four inch wafer to 700.degree. C. in three seconds, maintaining the temperature for ten seconds, and then ramping the temperature down in three seconds. Arai et al. reported applying 1600 watts to each of the lamps in the radiant heat source to heat a silicon wafer of 450 .mu.m in thickness and 4 inches square in area to a temperature of 1200.degree. C. within 10 seconds of when power was applied to the lamps.
In yet another apparatus for heating a semiconductor wafer, Robinson et al., U.S. Pat. No. 4,789,771, a wafer is supported above a susceptor in a reaction chamber. Infrared heat lamps extend directly through the reaction chamber. This design suffers from several shortcomings. The radiant heat lamps are exposed to the gases in the reaction chamber allowing deposits to form on the lamps. Additionally, the only cooling mechanism for the lamps and the inner surface of the reflectors is the gas flow through the chamber; consequently, lamp lifetime is probably adversely affected. Further, the reflectors are apparently at an elevated temperature, as well as the quartz sheets around the radiant energy bulbs so that, over time, deposits are formed on the bulb and reflector surfaces which, in turn, will affect the uniformity of layers formed on the susceptor. Last, special mechanisms are required to uniformly heat the susceptor surface because the susceptor rotation mechanism, which is typically opaque to radiant energy, prevents direct heating of the entire lower surface of the susceptor.